Certain organisms including plants and some microalgae use a type II fatty acid biosynthetic pathway, characterized by the use of discrete, monofunctional enzymes for fatty acid synthesis. In contrast, mammals and fungi use a single, large, multifunctional protein.
Type II fatty acid biosynthesis typically involves extension of a growing acyl-ACP (acyl-carrier protein) chain by two carbon units followed by cleavage by an acyl-ACP thioesterase. In plants, two main classes of acyl-ACP thioesterases have been identified: (i) those encoded by genes of the FatA class, which tend to hydrolyze oleoyl-ACP into oleate (an 18:1 fatty acid) and ACP, and (ii) those encoded by genes of the FatB class, which liberate C8-C16 fatty acids from corresponding acyl-ACP molecules.
Different FatB genes from various plants have specificities for different acyl chain lengths. As a result, different gene products will produce different fatty acid profiles in plant seeds. See, U.S. Pat. Nos. 5,850,022; 5,723,761; 5,639,790; 5,807,893; 5,455,167; 5,654,495; 5,512,482; 5,298,421; 5,667,997; and 5,344,771; 5,304,481. Recently, FatB genes have been cloned into oleaginous microalgae to produce triglycerides with altered fatty acid profiles. See, WO2010/063032, WO2011/150411, WO2012/106560, and WO2013/158938.